1. Field of the Invention
The present invention relates to a semiconductor absolute pressure sensor that detects the pressure change to be measured by the change in capacitance, and in particular to improving the electrode structure thereon, and increasing the leakage resistance of the reference pressure cavity that is indispensable to the absolute pressure sensor.
2. Description of Related Art
In a semiconductor pressure sensor that measures absolute pressure, the sealing characteristic of the reference pressure cavity 5 formed between a silicon substrate 3 having a diaphragm 1 and a silicon wall 2 and another substrate 4 bonded at the region of the silicon wall 2 of this silicon substrate 3, as shown for example in FIG. 4, is important. The reason is that in the case of a semiconductor pressure sensor, the deflection of this diaphragm depends on the pressure difference produced between the externally applied pressure and the inside of the reference pressure cavity. Therefore, when a leakage from the outside into the reference pressure cavity occurs, because the pressure in the reference pressure cavity gradually changes or fluctuates, even when the pressure applied from the outside is constant, the pressure value output by the sensor varies with time. In addition, this fluctuation depends on the rate of the leakage.
As shown in FIG. 4, a resistor 6 is formed in the diaphragm 1, and in the case of a semiconductor pressure sensor (referred to hereinbelow as a xe2x80x9cpiezoresistant pressure sensorxe2x80x9d) that detects the pressure of a measured medium based on the change in the resistance of the resistor 6, generally a hermetically sealed reference pressure cavity 5 is easily obtainable, and the leakage rate is small. The reason is that the detection circuit 7 for detecting the variation of this resistance can be formed on the surface of a silicon substrate 3, and can be formed completely independently of the reference pressure cavity 5. That is, normally, in a piezoresistant pressure sensor, the reference pressure cavity is sealed with a high degree of airtightness by the application of a widely used technology such as electrostatic bonding because there is no part where an electrode is brought out to the outside, etc., from within the reference pressure cavity.
In contrast, in the case of capacitance pressure sensor, because the pressure of the measured medium is detected based on the fluctuation in the capacitance between the diaphragm that deflects according to the pressure and the opposite substrate, as shown in the representative structure of FIG. 5, it is necessary to provide electrodes 8, 9 respectively on the diaphragm 1 side and the facing substrate 4 side. In addition, normally, one of the electrodes 8, 9 has a structure covered by an insulating layer 10 (the electrode 9 on the substrate side in FIG. 5).
Generally, in a semiconductor pressure sensor, the diaphragm is has as a starting material a silicon substrate. Therefore, on the diaphragm side, several technologies are used that form an electrode for capacitance detection by providing a high conductivity by doping in high concentration boron or phosphorus in the silicon substrate. In addition, when doping technology is used, a lead that connects the electrode in the reference pressure cavity and the external electrode can be easily obtained.
In addition, in FIG. 5, on substrate 4 opposite the diaphragm, glass substrates are widely used. The reason is that a glass substrate can be hermetically sealed to the silicon substrate at low cost by electrostatic bonding. However, on this glass substrate, because it is not possible to apply this doping technology, on the electrode 9 on the glass substrate 4 side, conventionally thin metallic layer has come to be used.
Here, in order to detect fluctuation in the capacitance, it is necessary to extend an electrode 9 on the glass substrate 4 side positioned within the reference pressure cavity 5 to an external electrode. (Hereinbelow, this lead is referred to as a feedthrough.)
The formation cost of the feedthrough is least expensive if it is formed at the same time as the electrode 9. Therefore, the thin metallic layer of the feedthrough comprises the same material as the electrode 9, and usually has the same thickness. In addition, the pattern width of the feedthrough should be as narrow as possible in order to decrease the parasitic capacity of the pressure sensor as a whole. However, in contrast, in order to make the electrical resistance small, the pattern width should be as wide as possible. Thus, the balance between both the parasitic capacity and the electrical resistance determines the width of the feedthrough.
The structure formed by this feedthrough and the external electrode is shown in FIG. 6 to FIG. 8. FIG. 6 is a vertical cross-section of the entire semiconductor pressure sensor, FIG. 7 is a planar drawing showing the electrode on the glass substrate side, and FIG. 8 is a planar drawing showing the electrode on the silicon substrate side. In these figures, reference numeral 9 is the electrode on the glass substrate side, 11 is the external electrode on the glass substrate side, 12 is the feedthrough that connects the electrode 9 on the glass substrate side and the external electrode 11, 8 is an electrode on the silicon substrate side, 13 is an external electrode on the silicon substrate side, and 14 is a lead that connects the electrode 8 on the silicon substrate side and the external electrode 13.
In the structure shown in FIG. 6 to FIG. 8, when the silicon substrate 3 and the glass substrate 4 undergo electrostatic bonding, as shown in FIG. 9, a difference in level is produced on the insulating layer 10 reflecting the difference in level produced by the feedthrough 12, and as a result, gaps 15 are made between the difference in level of the insulating layer 10 and the silicon substrate 3. This gap 15 becomes a leakage path between the outside of the sensor and the reference pressure cavity 5.
Here, as a means of excluding this leakage path, U.S. Pat. No. 5,528,452 by Wen K. Ko, proposes limiting the thickness of the metallic layer of the feedthrough to 0.1xcx9c0.3 xcexcm. It is disclosed that by making the feedthrough have this thickness, leakage to the reference pressure cavity is not produced.
However, when manufacturing a sensor having the extremely low leakage rate equal to or below 1xc3x9710xe2x88x9213 atmxc2x7cc/sec, even if the thickness of the metallic layer of the feedthrough is made 0.1 xcexcm, in the structure of the conventional sensor shown in FIG. 6 to FIG. 8, there is the major problem that the rate of defects due to this leakage exceeds 50%.
This phenomenon is caused by various changes in the size of the leakage path shown in FIG. 9 due to variations in the surface roughness of the glass substrate, or variations in the thickness of the metallic layer of the feedthrough, or further, variations in the thickness of the insulating layer, etc., and when this path exceeds a certain size, the leakage rate to the reference pressure cavity exceeds 1xc3x9710xe2x88x9213 atmxc2x7cc/sec.
In consideration of the above-described problem, the present invention has as an object providing a pressure sensor that prevents leakage to the reference pressure cavity from outside the sensor in a capacitor pressure sensor, and that has a structure which can decrease defects due to leakage in comparison with conventional technology.
In order to attain the above object, in a pressure sensor of the present invention that bonds the upper surface of a first substrate a second substrate having a diaphragm and a thick part at this thick part, provides a first electrode covered by insulating layer positioned opposite to the diaphragm of the second substrate on the surface of the first substrate, and at the same time provides a second electrode on the diaphragm of the second substrate, and detects the pressure fluctuation to be measured by the change or fluctuation of the capacitance formed by the first electrode and the second electrode, and at the bonding region of the second substrate on the surface of the first substrate, wiring extending from the first electrode to an external electrode is formed, and an additional wiring portion extending in a direction intersecting the wiring is provided on the wiring.
In this case, preferably the width of an additional wiring portion is equal to or less than 50% the width of the wiring and the length of the ridge is equal to or greater than the width of the wiring.
Here, the first substrate is, for example, a glass substrate, and the second substrate is, for example, as silicon substrate having a diaphragm and a thick part. The first electrode and second electrode are electrodes that form capacitance by being disposed such that each substrate faces each other, and the wiring, called the feedthrough in conventional technology, electrically connects the first electrode and the external electrode.
In addition, in another pressure sensor of the present invention, wiring is formed extending from the first electrode to an external terminal on the bonding region of the second substrate to the upper surface of the first substrate, and a frame crossing the wiring surrounding the circumference of this first electrode is provided on the wiring.
In this case, it is preferable that the wire width of this frame be equal to or greater than 2 xcexcm.
In the pressure sensor of the present invention, by forming an additional wiring portion or a frame that crosses the wiring, on the straight part of the wiring not forming the additional wiring portion and the frame, a leakage path such as that shown by reference numeral 15 in FIG. 9 is formed, but this leakage path is blocked by the insulating layer at the part forming the additional wiring portion or frame, and the gas molecules that penetrate through this path are blocked from the reference pressure cavity. Therefore, in the sensor with a structure having an additional wiring portion or a frame, the rate that gas diffuses to the reference pressure cavity from outside the sensor is extremely slow. That is, the sensor of the present invention can make the leakage rate to the reference cavity sufficiently low in comparison with the sensor having a conventional structure, and because the pressure fluctuation in the reference pressure cavity when a constant external pressure is applied is small, it is possible to realize a pressure sensor having a small leakage defect rate.
In addition, the preferred range of the dimensions (length, width) of the additional wiring portion and the frame are described below in the preferred embodiments.
According to the pressure sensor of the present invention, by an additional wiring portion or frame being formed crossing the wiring, the leakage path along the edge of the wiring is blocked by the insulating layer at the part formed by the additional wiring portion and frame, and the gas molecules that penetrate through this path are blocked from the reference pressure cavity. Therefore, in the sensor of the present invention, in comparison with the conventional sensor, it is possible to sufficiently reduce the leakage rate to the reference pressure cavity, and the pressure fluctuation within the reference pressure cavity when a constant external pressure is applied becomes small. Thereby, it is possible to realize a pressure sensor whose leakage defect rate is small.